On the limitations of using content classification as a crime fighting technique

Originally posted as a comment here.

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There are some key differences between the European filters and the proposed Australian filter.

First, the European filters are all currently voluntary. Second, the European filters all approach the problem of child abuse as an extension of law enforcement efforts. The approach for the Australian filter is quite different: it is trying to tackle child abuse as if it was a content classification problem.

Whereas the decision procedure for the European filter is: "is the material to be blocked illegal", the question for the Australian filter is: "has this material been refused classification by the body that assesses film and literature".

One would have thought that the European filters are at least tackling a criminal problem as a criminal problem. The Australian Government is attempting to tackle child abuse as a problem of good or bad taste.

Any guesses which approach might be more effective?